Explanation:
These two moons change places.
Epimetheus and
Janus,
two small moons of
Saturn,
actually switch positions as they orbit their home planet.
The orbital radii of the moons are strangely separated by less than the
radii of the moons themselves: about 50 kilometers.
One moon orbits Saturn well ahead of the other, at first.
As the two moons gravitationally attract, they approach each other and, every few years, actually
pass and trade orbits.
This strange dance creates speculation that
Epimetheus and Janus were once joined and later split from each other.
Pictured above,
the two moons were photographed rounding their orbits just outside of
Saturn's F ring.
The above image was taken in early September by the robot
Cassini spacecraft,
also orbiting Saturn.